Somatostatin is a 14-amino acid peptide hormone (somatostatin-14; SS-14) found in many cells, particularly those of neuroendocrine origin, that acts as a neurotansmitter in the central nervous system. Reubi et al., Cancer Res. 47, 5758-64 (1987). There is also a somatostatin variant released by β cells in the pancreatic islets that is a 28 amino acid peptide (somatostatin-28; SS-28). Somatostatin has an inhibitory effect on growth hormone, and a generally antiproliferative effect. Somatostatin receptors (SSTR or SSR) are found on the surface of human tumor cells, including cells with amine precursor uptake and decarboxylation properties, such as pituitary tumors, endocrine pancreatic tumors, carcinoids, paragangliomas, small cell lung cancers, medullary thyroid carcinomas and pheochromocytomas. Reubi et al., Metabolism, 41, 104-10 (1992); Patel, Front. Neuroendocrinol. 20, 157-98 (1999). Somatostatin receptors belong to the guanine nucleotide-binding regulatory protein (G-protein)-linked receptor family.
Synthetic somatostatin analogs such as octreotide and lanreotide have been used for antitumor treatment and cancer detection. Jensen et al., J. Clin. Endocrinol. Metab., 85(10), 3507-8 (2000). Analogs of somatostatin were developed because human somatostatin has a very short half-life in circulation (2-3 minutes) and is easily broken down by endogenous peptidases. Rens-Domiano et al., J. Neurochem, 58, 1987-96 (1992). Somatostatin analogs typically, but need not, retain two important molecular features of somatostatin: its cyclic form and the 4 amino acids involved in the binding to the somatostatin receptor (i.e., amino acids 7-10 of the somatostatin sequence). A number of radiolabeled somatostatin analogs (e.g., [111In-DTPA-DPhe1]octreotide) have been developed that can be used to image these tumors using somatostatin receptor scintigraphy. Krenning et al., Bur. J. Nucl. Med., 20, 716-731 (1993). Somatostatin receptor scintigraphy is the most sensitive method to localize the primary and metastatic disease in subjects with all pancreatic endocrine tumors and carcinoids. Gibril et al., Ann. Intern. Med. 125, 26-34 (1996). The localization of these tumors by somatostatin receptor scintigraphy is due to the interaction of the radiolabeled analogs with specific cell surface somatostatin receptors.
Multiple subtypes of somatostatin receptors are known, and almost all neuroendocrine tumors (carcinoids, pancreatic endocrine tumors) possess at least one subtype, frequently multiple subtypes. Somatostatin receptor subtypes (sst1, sst2, sst3, sst4, and sst5) have been isolated and cloned. Both octreotide and lanreotide have high affinity for somatostatin receptor sybtypes sst2 and sst5, lower affinity for sst3 and very low affinity for sst1 and sst4. Patel, Front. Neuroendocrinol. 20, 157-198 (1999). Radiolabeled analogs of octreotide are rapidly internalized and the radiolabeled peptides can remain present in the cells for prolonged periods. Hofland et al., Proc Assoc Am Physicians. 111:63-69 (1999).
Radiotherapy using high doses of [111In-DTPA-D-Phe1]octreotide (DTPA: diethylenetriaminepentacetic acid), which emits auger and conversion electrons, as well as 90yttrium-labeled somatostatin analogs coupled by a DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) chelator, which can emit β-particles and give high radiation doses of greater penetrance, have been reported to inhibit tumor growth in both animal studies and in preliminary human studies. deJong et al., Q. J. Nucl. Med. 43, 356-366 (1999). Additional examples of somatostatin analogs that are being evaluated for use in the radionuclide therapy of tumors include [DOTA0, Tyr3] octreotide (DOTATOC) labeled with 131I, 90Y or 177Lu. Peptide receptor radionuclide therapy (PRRT) using radiolabeled DOTATOC has led to tumor responses in the majority of subjects, but has also posed problems with regard to renal and hematological toxicity. Reubi, Endocr. Rev. 24, 389-427 (2003). Another synthetic somatostatin-receptor targeting analog, [DOTA0, Tyr3]octreotate (DOTATATE) labeled with 177Lu has recently been investigated for PRRT. J, Nucl. Med. 2005 January; 46 Suppl 1:107S-14S; J Nucl Med. 2005 January; 46 Suppl 1:83S-91S1 Endocr Relat Cancer. 2005 December; 12(4):683-99.
Despite good imaging and diagnostic results with 111In labeled [DTPA0] octreotide (Octreoscan®) in the last few years, there have been several reports describing new somatostatin radioligands for studying sst expression. Some like [DOTA0, Tyr3] octreotide (DOTATOC) labeled with 131I, 90Y and 177Lu are also being evaluated for use in the radionuclide therapy of tumors (7). The new Peptide Receptor Radionuclide Therapy (PRRT) using radiolabeled DOTATOC has led to tumor responses in the majority of patients, but has also posed problems with renal and hematological toxicities Reubi, Endocr. Rev. 24, 389-427 (2003). In previous studies, kidney failures have been reported after treatment with DOTATOC labeled to β− particle emitter 90Y (8-10). In previously completed clinical studies, it was observed that 10% to 34% patients had complete remission following 90Y-DOTATOC treatment (11). The results of these studies illustrate the partial treatment potentials of this agent and the possible higher relapse rates that may occur in the future (12). The primary challenges that 90Y or 177Lu labeled DOTATOC faces are renal toxicities and incomplete treatments, especially in radio-resistant tumors.
Recent studies indicate that the presence of somatostatin receptors on other more common non-endocrine tumors may also be used for tumor localization or treatment. Halmos et al., J Clin Endocrinol Metab., 85, 3509-12 (2000). Increased densities of somatostatin receptors are found in various tumors of the central nervous system (meningiomas, astrocytomas, gliomas), some malignant lymphoid tumors (Hodgkin's disease, non-Hodgkin's disease), and in some cancers of the prostate, breast, kidney, liver, and lung. Jensen et al., J. Clin. Endocrinol. Metab., 85(10), 3507-8 (2000). Somatostatin analogs have been shown to have antiproliferative effects on breast, gastric, colorectal, prostate, thyroid and lung tumors, and cytotoxic somatostatin analogues have been shown to inhibit growth of human breast cancer, prostate cancer, renal cell carcinomas, and human glioblastomas. Kath et al., Recent Results Cancer Res. 153, 23-43 (2000); Froidevaux et al., Curt. Med. Chem. 7, 971-994 (2000). The effect of chemotherapeutic agents on the expression of somatostatin receptors has been investigated using pancreatic tumor cells. Fueger et al., J. Nucl. Med. 42(12), 1856-62 (2001).
Gemcitabine (2′,2′-difluoro-2′-deoxycytidine; dFdC) is a pyrimidine analog that has shown activity in various solid tumors, including non-small cell lung cancer (NSCLC), small cell lung cancer, head and neck squamous cell cancer, germ cell tumors, lymphomas (cutaneous T-cell and Hodgkins' disease), mesothelioma, and tumors of the bladder, breast, ovary, cervix, pancreas, and biliary tract, as well as some hematologic malignancies. The compound was first reported by Lilly Research Laboratories, Eli Lilly and Co.; Indianapolis, Ind. Hertel et al., Cancer Res. 50, 4417-4422 (1990). Gemcitabine is a deoxycytidine analog with structural similarities to cytarabine (Ara-C).
Gemcitabine is metabolized intracellularly by nucleoside kinases to the active diphosphate (dFdCDP) and triphosphate (dFdCTP) nucleoside metabolites. The cytotoxic effect of gemcitabine is generally attributed to the actions of diphosphate and the triphosphate nucleosides, which lead to inhibition of DNA synthesis. Gemcitabine diphosphate (dFdCDP) inhibits ribonucleoside reductase, which is responsible for catalyzing the reactions that generate the deoxynucleoside triphosphates for DNA synthesis. Inhibition of this enzyme by the diphosphate nucleoside causes a reduction in the concentration of the deoxynucleotides, including dCTP. Gemcitabine triphosphate (dFdCTP) competes with dCTP for incorporation into DNA. The reduction in the intracellular concentration of dCTP (by the action of the diphosphate) further enhances the incorporation of gemcitabine triphosphate into DNA, a process referred to as self-potentiation. After the gemcitabine nucleotide is incorporated into DNA, only one additional nucleotide is added to the growing DNA strand. Further DNA synthesis is inhibited, as DNA polymerase epsilon is unable to remove the gemcitabine nucleotide and repair the growing DNA strand, resulting in what is known as masked chain termination. Gemcitabine induces an S-phase arrest in the cell cycle, and triggers apoptosis in both human leukemic cells and solid tumors. Tolis et al., Eur. J. Cancer, 35, 797-808 (1999). In addition to its cytotoxic effect, gemcitabine is a potent radiosensitizer. Gemcitabine has been investigated as a radiosensitizer for rodent and human tumor cells, including those found in pancreatic, non-small cell ung, head and neck, colorectal, breast, and cervical cancer. Pauwels at al., Oncologist 10(1), 34-51 (2005).